Acid & Metalliferous Drainage Assessment
The range of water quality issues that are related to the oxidation of sulfide minerals are variably known as:
- Acid & metalliferous drainage (AMD),
- Acid mine drainage (AMD),
- Acid rock drainage (ARD),
- Neutral metalliferous drainage (NMD),
- Neutral mine drainage (NMD),
- Metal leaching / Acid rock drainage (ML / ARD).
One of the early environmental tasks for exploration or mining companies is to assess the risk posed by their mining wastes and other geological materials on water quality. Geological materials commonly include waste rock, tailings, ore, concentrate, pit wallrock, heap leach materials and slag. The risk assessment process commences with strategic sampling and analysis of various geological materials for static geochemical parameters. It is vital that a suitable density of representative samples is selected for this initial analytical program. Sampling requirements will vary between greenfield and brownfield sites.
Static geochemical testwork quantifies the maximum potential acidity that a geological material can produce, as well as its maximum acid neutralisation capacity. Such analytical programs provide a fundamental characterisation of the behaviour of geological materials, but offer no insight into the rates of acid generation or the effectiveness of acid neutralisation. Static geochemical characterisation programs typically include the following analytical parameters:
- Total Sulfur
- Chromium-reducible Sulfur
- NAPP (net acid producing potential)
- ANC (acid neutralisation capacity)
- NAG pH
- NAG 4.5
- NAG 7.0
- pH KCl
- Titratable Actual Acidity (TAA)
- pH ox
- Titratable Peroxide Acidity (TPA)
- Total / Organic / Inorganic Carbon
In combination, these analytical parameters facilitate a high-level assessment of the bulk geochemical properties and behaviour of geological materials containing reactive sulfides and carbonates. Careful assessment of such data enables the AMD/ARD risk of each sample to be classified as either Potentially Acid Forming (PAF) or Non Acid Forming (NAF). More complex Classification systems can also be developed from these static characterisation programs when appropriate.
The primary aim of all Classification systems is to facilitate the formulation of an environmental geochemical layer in the Mine Block Model.
Environmental Geochemical Layer for your Mine Block Model
There are several methods of producing an environmental geochemical layer for a mine block model without needing to characterise several thousand samples. Some of these methods are straightforward, and others can be quite complex. In most situations you will likely need the support of a geochemical specialist.
The key driver for developing a mine block model layer is to have the ability to formulate a waste schedule that informs operations about the AMD/ARD risk (PAF/NAF) of mine wastes arriving at waste rock storage facilities or tailings storage facilities on a weekly or monthly basis. Without such knowledge, it is very difficult to manage site water quality during operations or post closure. For example, it is important to identify and use geochemically benign materials for construction purposes (TSF embankments, roads, foundations), and it is also difficult to design and construct non-polluting waste rock piles without strategic placement of inert and reactive materials. By combining knowledge of the geochemical properties (AMD/ARD risk layer in your mine block model) of your waste materials with information on which rock types will be mined over the coming weeks and months (mining schedule), it is possible to devise a schedule of NAF and PAF lithologies that are to be excavated from your open cut or underground mine into the future. This knowledge provides high level control on the ideal placement of problematic materials in advance of mining, and is vital for effective AMD/ARD management.
Depending on the resolution provided by your environmental geochemical layer, additional geochemical assays may be necessary to define AMD/ARD risk on a truck-by-truck basis. For example, assay data from blast holes on each bench can include static geochemical parameters that permit definitive classification (AMD/ARD risk assessment).
Up to this stage we have established whether your waste materials are non acid forming (NAF) or potentially acid forming (PAF), and when these materials are likely to be encountered during mining. While this is key information, we still do not know “how long the AMD/ARD will continue for,” “what the rate of acid generation will be,” or “how long could the acid forming processes be retarded by the natural acid neutralising capacity of the materials” (ie. lag time before acidification). These questions significantly affect operational and post closure decisions. In addition, at this point in time we generally do not understand what type of water quality / chemistry impacts could be associated with oxidation of your mine wastes. Kinetic testwork procedures have been designed to answer these questions.
There are three different types of routine kinetic testwork procedures, (i) column leach, (ii) humidity cell leach and (iii) oxygen consumption (OxCon). Each of these approaches offers strengths and weaknesses and the choice of the most appropriate method can come down to time, cost and site-specific chemistry issues. For tailings materials, which commonly demonstrate relatively unusual physical properties, a new and innovative kinetic testwork approach referred to as “oxygen penetration testwork” (OPT) is being used more commonly to deal with their unique oxidation behaviour.
Sulfide oxidation rates can be derived indirectly from column leach and humidity cell leach testwork, or directly from oxygen consumption testwork. Such data, when combined with the known or planned tonnages of different geologic materials, can be used to predict acidity fluxes from different mine site domains (eg. waste rock piles, tailings dams, heap leach pads, ore stockpiles). Such predictions can be used to focus environmental design or remediation efforts on specific domains. If initial indications are that waste rock piles are likely to represent the biggest AMD/ARD risk on site, both during operations and post closure if no special management action is taken, then this domain needs to form the focus of design and construction efforts to prevent or minimise AMD/ARD.
AMD / ARD Management
The only reason for all efforts to this point, including static geochemical characterisation, AMD/ARD risk classification, environmental layer development for the mine block model, kinetic testwork and prediction, is to provide the tools for preventing, avoiding, minimising or controlling AMD/ARD issues. It’s all about gathering the data to facilitate AMD/ARD management.
There were very few management options 30 years ago, but today there is a broad range of effective strategies.